Hydrogen-absorbing alloy of ultra high capacity for electrode of secondary battery

ABSTRACT

The present invention provides a hydrogen-absorbing alloy system of ultra high capacity for electrode of secondary battery. In accordance with the present invention, the hydrogen-absorbing Ti alloy system is represented as a following general formula: 
     
         Ti.sub.A Zr.sub.B V.sub.C Mn.sub.D Ni.sub.E M.sub.F 
    
     wherein, M represents at least one metal which is selected from the group consisting of Cr, Co, Fe, Cu, Al, Si, Hf, Nb, Mo and R.E., where R.E. represents at least one metal which is selected from the group of rare-earth elements consisting of La, Ce, Pr, Nd and Sm; and, A, B, C, D, E and F have atomic ratios ranging 0.2≦A≦0.35, 0.03≦B≦0.15, 0.15≦C≦0.4, 0.8≦D≦0.2, 0.13≦E≦0.35 and 0≦F≦0.1, respectively, with the proviso that A+B+C+D+E+F=1 and A+B≦0.45. The hydrogen-absorbing Ti alloy system of the invention, has molar molecular weight of 50 to 65 g/mol, C14-hexagonal crystalline structure of single phase, lattice constant of a: 4.902-5.004 Å and c: 7.972-8.168 Å, ultra high discharge capacity of 400 mAh/g or more, which can be employed as an anode material of a Ni--MH secondary battery.

FIELD OF THE INVENTION

The present invention relates to a hydrogen-absorbing Ti alloy system,more specifically, to a hydrogen-absorbing Ti alloy system havingC14-hexagonal crystalline structure of single phase and ultra highdischarge capacity, which can be employed as an anode material of aNi--MH(nickel-metal hydride) secondary battery.

BACKGROUND OF THE INVENTION

In general, a Ni--MH secondary battery in which a hydrogen-absorbingalloy is employed as an anode material, works on a reaction principle asfollowings: In the course of electric: discharge of a battery, hydrogenatoms within the hydrogen-absorbing alloy bind to OH⁻ ions of KOHelectrolyte to give water, and electrons simultaneously move to acathode through external circuit. During electric charging, water iselectrolysed to give H⁺ and OH⁻ ions, where OH⁻ ions stay in theelectrolyte, and H⁺ ions bind to influxed electrons to give hydrogenatoms, which, in turn, bind to the hydrogen-absorbing alloy finally tobe stored within the alloy. This reaction occurred, based on theproperties of hydrogen-absorbing alloy that it is stable in an alkalinesolution and absorbs/releases a lot of hydrogen rapidly and reversibly.

In order to use the hydrogen-absorbing alloy system as an anode materialof a Ni--MH secondary battery, the alloy system should meet followingrequirements: First, the hydrogen-absorbing alloy has to possesshydrogenation properties such as proper hydrogen absorption-desorptionpressure in a solid-gas reaction (generally, 0.01 to 1 atmosphere atroom temperature), rapid hydrogenation rate, and a highhydrogen-absorbing capacity(a theoretical discharge capacity of anelectrode is proportional to a hydrogen-absorbing capacity (C_(H) (wt.%)): a theoretical discharge capacity (mAh/g)=268×C_(H))). Secondly,charge transfer associated with the oxidation and reduction of hydrogenat the interface between the alloy and the KOH electrolyte, has to occureasily during the electrochemical reaction of the alloy and theelectrolyte. Accordingly, only the hydrogen-absorbing alloy whosesurface functions as a catalyst for charge transfer reaction can be usedas an anode material of a Ni--MH secondary battery.

So far, many hydrogen-absorbing alloy systems satisfying the saidrequirements have been reported, whose examples includes: AB₅type-hexagonal structure of La--Nd--Ni--Co--Al alloy system (see: U.S.Pat. No. 4,488,817), Mm--Mn--Ni--Co--Al alloy system (see: JP61-1132501; JP 61-214361); Ti--V--Ni--Cr alloy system of AB₂type-C14,15-hexagonal and BCC (body-centered cubic lattice) multiphasestructure (see: U.S. Pat. No. 4,551,400); Zr--V--Ni alloy system of C14structure (see: J. of the Less-Common Metals, 172-174:1219 (1991)); and,Zr--Cr--Mn--Ni alloy system of C14, C15 structure (see: J. of theLess-Common Metals, 172-174:1211(1991)).

Among the said alloy systems, La-Ni electrode of AB₅ type shows highlyreduced capacity during charge/discharge cycling in an alkalineelectrolyte (see: J. of the Less-Common Metals, 161:193(1990); J. of theLess-Common Metals, 155:119(1989)), which is called as "degradation". J.J. G. Willems et al. have substituted a small amount of Ni with Co, Al,and a small amount of La with Nd to increase durability forcharge/discharge cycling, while it results in the reduction of capacity(see: U.S. Pat. No. 4,488,817).

On the contrary, it has been reported that electrolesshydrogen-absorbing alloy powder with copper results in an increase inthe durability for charge/discharge cycling without the reduction ofcapacity (see: J. of the Less-Common Metals, 107:105(1985)). However,the said method essentially accompanies a step of plating andenvironmental pollution caused by solutions employed therein.

On the other hand, it has been found that hydrogen-absorbing alloy ofAB₂ type has a discharge capacity of 300 mAh/g or more, which is higherthan that of AB₅ type alloy, and has a good durability forcharge/discharge cycling without a step of plating (see: J. of theLess-Common Metals, 172-174:1175(1991); J. of the Less-Common Metals,180:37(1992)).

In addition, T. Gamo et al. discloses a hydrogen-absorbing Z-based alloysystem which comprises Zr over 30 at. % and Ni over 40 at. % and has adischarge capacity of 300 to 370 mAh/g (see: U.S. Pat. No. 4,946,646).Also, K. Hong and M. A. Fechenko et al. teaches a hydrogen-absorbingalloy system of Ti--Zr--V--Ni--Cu--Mn--M (M═Al, Co, Fe, etc.) having adischarge capacity of 300 to 380 mAh/g (see: U.S. Pat. No. 4,849,205;U.S. Pat. No. 4,728,586; U.S. Pat. No. 4,551,400).

All of the said conventional hydrogen-absorbing alloys are, however,proven to be less satisfactory in the sense that they have dischargecapacities of 250 to 320 mAh/g (AB₅ type) and 300 to 380 mAh/g (AB₂type), which are lower than 400 mAh/g.

Recently, in accordance with the advent of electric vehicles andelectronic machines such as cellular phone, notebook computer andcamcorder, etc., and tendency of miniaturization of the electronicmachines, there are strong reasons for exploring and developingalternative batteries of high capacity. However, a hydrogen-absorbingcapacity of AB₅ type alloy can not reach to 1.2 wt % (a dischargecapacity of 320 mAh/g) or more, since the alloy has a molar ratio ofA_(1/6) B_(5/6) and a molar molecular weight of 72 g/mol. Also, ahydrogen-absorbing capacity of Z-based alloy system, among the AB₂ typealloys, can not reach to 1.4 wt % (a discharge capacity of 400 mAh/g) ormore, since the alloy has a molar ratio of A_(1/3) B_(2/3) and a molarmolecular weight of 67 g/mol. Therefore, needs have continued to existfor the development of a new hydrogen-absorbing alloy having a dischargecapacity over 400 mAh/g, since further increase in the energy density ofa battery can not be expected in the conventional hydrogen-absorbingalloys.

Accordingly, the hydrogen-absorbing alloy of light weight has to be usedin order to down the molar molecular weight of the hydrogen-absorbingalloy to be under 65 g/mol, whose examples include alloy systemscomprising Mg, V and Ti elements. However, the Mg alloy system can notbe used for an electrode, since it has lowhydrogen-absorption/desorption pressure at room temperature and slowhydrogenation rate. Although the V alloy system, which is substitutedwith a small amount of Ti and Zr, has properhydrogen-absorption/desorption pressure of 0.01-1 atm at roomtemperature, it can not be used for an electrode, since it can notfunction as a catalyst of charge transfer reaction.

On the other hand, the hydrogen-absorbing Ti alloy system has beenevaluated as a proper hydrogen-absorbing material, since it has a largehydrogen-absorbing amount(about 1.96 wt % H₂ /alloy(g)) and highreaction rate. In this connection, T. Gamo suggests a hydrogen-absorbing Ti alloy system (see: U.S. Pat. No. 4,144,103; U.S. Pat. No.4,160,014), however, it has a relatively high hydrogen equilibriumpressure of 5 to 10 atm and whose surfaces can not function as catalystsof charge transfer reaction in KOH electrolyte. Therefore, the alloysystem can not be practically applied as an anode material of asecondary battery.

SUMMARY OF THE INVENTION

The present inventors have made an effort to solve the said problems ofthe conventional hydrogen-absorbing Ti alloy systems. They carried outpartial substitution of Ti with V and Zr in a hydrogen-absorbing Ti--Mnalloy system to increase a hydrogen-absorbing capacity and controlhydrogen-absorption/desorption equilibrium pressure to be 0.01 to 1 atm.In addition, they added a small amount of Ni or partially substituted Mnwith Ni to improve the stability in KOH electrolyte and the catalyticfunction, and further added at least one metal which is selected fromthe group consisting of Cr, Co, Fe, Cu, Al, Si, Hf, Nb, Mo andR.E.(rare-earth elements), as the 6th and 7th elements in a smallamount. Thus, they finally obtained a hydrogen-absorbing alloy systemwhich meets two requirements of an electrode material, i.e., properhydrogenation properties in a solid-gas reaction and properelectrochemical reaction within the electrolyte for a anode electrode ofa Ni--MH secondary battery, which has C14-hexagonal crystallinestructure of single phase, proper hydrogen-absorption/desorptionequilibrium pressure, catalytic function of charge transfer reaction,high hydrogen-absorbing capacity, high reaction rate and light weight,which can be used as an anode material of a Ni--MH secondary battery.

A primary object of the present invention is, therefore, to provide anovel hydrogen-absorbing Ti alloy system having C14-hexagonalcrystalline structure of single phase and ultra high discharge capacity,which can be employed as anode material of a Ni--MH secondary battery.

BRIEF DESCRIPTION OF DRAWINGS

The above and the other objects and features of the present inventionwill become apparent from the following description given in conjunctionwith the accompanying drawings, in which:

FIG. 1(A) is a graph showing X-ray diffraction pattern for Ti₀.4 ZrO₀.05V₀.2 Mn₀.2 Ni₀.2 alloy system.

FIG. 1(B) is a graph showing X-ray diffraction pattern for Ti₀.4 V₀.2Mn₀.2 Ni₀.2 alloy system.

FIG. 1(C) is a graph showing X-ray diffraction pattern for Ti₀.4 V₀.2Mn₀.4 alloy system.

FIG. 2 is a graph showing X-ray diffraction pattern for Ti₀.26 Zr₀.07V₀.15 Mn₀.2 Ni₀.34 alloy system.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, the hydrogen-absorbing Tialloy system is represented as a following general formula:

    Ti.sub.A Zr.sub.B V.sub.C Mn.sub.D Ni.sub.E M.sub.F

wherein,

M represents at least one metal which is selected from the groupconsisting of Cr, Co, Fe, Cu, Al, Si, Hf, Nb, Mo and R.E.,

where R.E. represents at least one metal which is selected from thegroup of rare-earth elements consisting of La, Ce, Pr, Nd and Sm; and,

A, B, C, D, E and F have atomic ratios ranging 0.2≦A≦0.35, 0.03≦B≦0.15,0.15≦C≦0.4, 0.08≦D≦0.2, 0.13≦E≦0.35 and 0≦F≦0.1, respectively,

with the proviso that A+B+C+D+E+F=1 and A+B≦0.45.

As a preferred embodiment of the present invention, a hydrogen-absorbingTi alloy system is represented as a following general formula(I):

    Ti.sub.A Zr.sub.B V.sub.C Mn.sub.D Ni.sub.E                (I)

wherein,

A, B, C, D and E have atomic ratios ranging 0.2≦A≦0.35, 0.03≦B≦0.15,0.15≦C≦0.4, 0.08≦D≦0.2 and 0.13≦E≦0.35, respectively, with the provisothat A+B+C+D+E=1 and A+B≦0.45.

Most preferred embodiments of the hydrogen-absorbing Ti alloy system ofthe invention are as follows:

Ti₀.20 Zr₀.13 V₀.20 Mn₀.17 Ni₀.30 ; Ti₀.20 Zr₀.14 V₀.20 Mn₀.11 Ni₀.35 ;Ti₀.23 Zr₀.10 V₀.20 Mn₀.17 Ni₀.30 ; Ti₀.26 Zr₀.07 V₀.24 Mn₀.20 Ni₀.23 ;Ti₀.27 Zr₀.07 V₀.17 Mn₀.17 Ni₀.32 ; Ti₀.27 Zr₀.07 V₀.18 Mn₀.17 Ni₀.31 ;Ti₀.28 Zr₀.07 V₀.22 Mn₀.18 Ni₀.25 ; Ti₀.30 Zr₀.30 V₀.17 Mn₀.17 Ni₀.33 ;Ti₀.30 Zr₀.03 V₀.20 Mn₀.17 Ni₀.30 ; and, Ti₀.35 Zr₀.04 V₀.18 Mn₀.30Ni₀.13.

As the other preferred embodiment of the present invention, ahydrogen-absorbing Ti alloy system is represented as a following generalformula(II):

    Ti.sub.A Zr.sub.B V.sub.C Mn.sub.D Ni.sub.E M.sub.F        (II)

wherein,

M represents at least one metal which is selected from the groupconsisting of Cr, Co, Fe, Cu, Al, Si, Hf, Nb, Mo and R.E.,

where R.E. represents at least one metal which is selected from thegroup of the rare-earth elements consisting of La, Ce, Pr, Nd and Sm;and,

A, B, C, D, E and F have atomic ratios ranging 0.2≦A≦0.35, 0.03≦B≦0.15,0.15≦C≦0.4, 0.08≦D≦0.2, 0.13≦E≦0.35 and 0<F≦0.1, respectively, with theproviso that A+B+C+D+E+F=1 and A+B≦0.45.

Most preferred embodiments of the hydrogen-absorbing Ti alloy system ofthe invention are as follows:

Ti₀.20 Zr₀.04 V₀.40 Mn₀.15 Ni₀.15 Co₀.05 ; Ti₀.26 Zr₀.07 V₀.24 Mn₀.10Ni₀.20 Cr₀.10 ; Ti₀.26 Zr₀.04 V₀.24 Mn₀.10 Ni₀.25 Hf₀.05 ; Ti₀.26 Zr₀.07V₀.24 Mn₀.10 Ni₀.25 Mo₀.05 ; Ti₀.26 Zr₀.07 V₀.24 Mn₀.10 Ni₀.25 Nb₀.05 ;Ti₀.27 Zr₀.07 V₀.21 Mn₀.14 Ni₀.27 Cr₀.04 ; Ti₀.27 Zr₀.07 V₀.21 Mn₀.14Ni₀.27 Co₀.04 ; Ti₀.27 Zr₀.07 V₀.21 Mn₀.14 Ni₀.27 Cu₀.04 ; Ti₀.27 Zr₀.07V₀.21 Mn₀.14 Ni₀.27 Al₀.04 ; and, Ti₀.27 Zr₀.07 V₀.21 Mn₀.14 Ni₀.27Si₀.04. Ti₀.27 Zr₀.07 V₀.21 Mn₀.14 Ni₀.27 Fe₀.04 ; Ti₀.27 Zr₀.07 V₀.21Mn₀.13 Ni₀.27 Co₀.06 ; Ti₀.28 Zr₀.07 V₀.20 Mn₀.15 Ni₀.25 R.E.₀.05(R.E.=rare-earth elements); Ti₀.28 Zr₀.07 V₀.22 Mn₀.10 Ni₀.25 Cr₀.04 ;and, Ti₀.28 Zr₀.07 V₀.22 Mn₀.10 Ni₀.25 Al₀.08.

The hydrogen-absorbing Ti alloy system of the invention hasC14-hexagonal crystalline structure of single phase, hydrogen-absorbingcapacity of Ni--MH 1.5 wt % or more, i.e., discharge capacity of 400mAh/g or more, and light molar molecular weight of 50 to 65 g/mol. Also,lattice constants of the hydrogen-absorbing alloy system of theinvention are as follows: a: 4.902-5.004 Å, c: 7.972-8.168 Å.

In the hydrogen-absorbing alloy system, Ti is a basic element, whoseoptimum atomic ratio ranges 0.2 to 0.35, considering the compositionratios of the substituted metal elements, i.e., V and Zr. If thecomposition ratio deviates from the optimum ratios, a hydrogen-absorbingcapacity decreases too much, or BCC phase increases, which, in turn,results in a decrease of a hydrogen-absorbing capacity.

In the hydrogen-absorbing alloy system, Zr whose optimum atomic ratioranges 0.03 to 0.15, leads to decrease in ratio of BCC phase andincrease in lattice size. If the composition ratio deviates from theoptimum ratio, the ratio of BCC phase determining crystalline structureof the alloy increases, which gives rise to a structure of multiphase,decreased discharge capacity under 400 mAh/g and increased hydrogenequilibrium pressure of above 1 atm.

In the hydrogen-absorbing alloy system, an optimum atomic ratio of Vranges 0.15 to 0.4. Since V has a low affinity for hydrogen, partialsubstitution of Ti with V leads to decreased hydrogen-absorbing amount,and hydrogen-absorbing capacity are increased, decreased andreincreased, which are caused by a relative ratio between BCC phase ofTi--V having no reversible hydrogen absorption/desorption activity andC-14 hexagonal structure of Ti--V--Mn having reversible hydrogenabsorption/desorption activity.

In the hydrogen-absorbing alloy system, an optimum atomic ratio of Mnranges 0.08 to 0.2. If the atomic ratio of Mn is lower than 0.08, aratio of BCC phase increases in the crystalline structure, which resultsin a remarkable decrease of the reversible hydrogen-absorbing capacity.If the atomic ratio of Mn is higher than 0.2, the composition ratio ofNi which is the substituted metal element, decreases, which results in adecrease of the stability in the KOH electrolyte and the catalyticfunction.

In the hydrogen-absorbing alloy system, Ni is an element conferring thestability in the KOH electrolyte and the catalytic function, whoseoptimum atomic ratio ranges 0.15 to 0.35.

The 6th and 7th elements can be further added to the hydrogen-absorbingalloy system of the invention, which are selected from the groupconsisting of Cr, Co, Fe, Cu, Al, Si, Hf, Nb, Mo and R.E. (rare-earthelements such as La, Ce, Pr, Nd and Sm), in a minimum amount under 10 wt%, so that BCC phase does not appear in the hydrogen-absorbing alloysystem.

Samples of the hydrogen-absorbing alloys of the invention are preparedand characterized.

(1) Method of securing hydrogenation properties in a solid-gas reaction

Based on the atomic ratios of elements composing the hydrogen-absorbingalloys, each elements were weighed for the total weight of 5 g, and arcmelted in an argon atmosphere. In this connection, in order to improvehomogeneity of the samples, they were overturned after congelation ofthe melted samples and melted again for 4 to 5 times. Then, the sampleswere ground and 100 to 200 mesh of the samples were added to thereaction tube, which was connected to the high-pressure hydrogenapparatus of Sievert's type. Activation was carried out by maintainingthe pressure within the reaction tube to reach the level of about 10⁻²torr for 30 minutes and adding hydrogen of about 20 atm without heattreatment, by which hydrogen absorption was completed in 1 hour. Then,the reaction tube was vacuumized to release hydrogen in the samples. Thehydrogen absorption-desorption was repeated 3 to 4 times to complete thestep in several minutes. After activation, a constant temperature in thehydrogen injection apparatus including the reaction tube was maintained,employing an automatic temperature controller. Then, a curve for thehydrogen equilibrium pressure in accordance with the hydrogencomposition during the hydrogen absorption-desorption at a temperaturewas obtained. Thus, thermodynamic properties such as hydrogen absorptioncapacity and plateau pressure, etc. were obtained from the curve.

(2) Method of measuring properties of the hydrogen-absorbing alloys inan alkaline electrolyte

Based on the desired composition, pure metals were weighed, and meltedin an argon atmosphere to prepare hydrogen-absorbing alloys. The alloysamples thus prepared were ground mechanically, mixed with copper ornickel powder and TEFLON powder, and moulded by pressing to prepareelectrodes. The electrodes were dipped in 30 wt % KOH electrolyte, andplatinum or nickel was used as a counter-electrode to construct ahalf-cell. Constant current was driven to run between the two electrodesemploying an ammeter, to inject hydrogen into the electrode. Duringelectric discharge, the two electrodes were exchanged each other, andcurrent was driven to run. In this connection, in order to measure avoltage of electrode of the hydrogen-absorbing alloy, a mercury oxideelectrode (Hg/HgO) was used as a reference electrode. A dischargecapacity is represented as ampere capacity per unit weight during thedischarge with a constant current.

The present invention is further illustrated by the following examples,which should not be taken to limit the scope of the invention.

Reference Example 1: Ti--V--Mn alloy system

In Ti--Mn alloy system, Ti was partially substituted with V elementhaving a low affinity for hydrogen (-37 kJ/mol), and hydrogenationproperties and microstructures of the alloys were observed, which weresummarized in Table 1 below.

                  TABLE 1    ______________________________________    Hydrogenation properties and microstructures in Ti--Mn and Ti--V--Mn    alloy systems            Hydrogen-absorbing            amount/amount of                       Plateau            reversible hydrogen                       pressure at                                 microstructure    Ti--V--Mn (wt %)       30° C. (atm)                                     BCC   C14    ______________________________________    Ti.sub.0.6 Mn.sub.0.4              2.76/0.39    0.1-2     70%   30%    Ti.sub.0.4 V.sub.0.2 Mn.sub.0.4              2.33/1.41    0.1-1     15%   85%    Ti.sub.0.35 V.sub.0.25 Mn.sub.0.4               2.1/0.75      0.1-0.7 67%   33%    Ti.sub.0.3 V.sub.0.3 Mn.sub.0.4              1.96/0.9       0.1-0.5 50%   50%    Ti.sub.0.25 V.sub.0.35 Mn.sub.0.4              1.92/0.8      0.01-0.2 60%   40%    Ti.sub.0.2 V.sub.0.4 Mn.sub.0.4               1.9/1.62    0.5-1      5%   95%    Ti.sub.0.3 V.sub.0.4 Mn.sub.0.3              2.45/0.2     0.1<      100%   0%    Ti.sub.0.4 V.sub.0.3 Mn.sub.0.3              1.9/0.5      0.1<      100%   0%    ______________________________________

As can be seen in Table 1, partial substitution of Ti with V led to adecrease of hydrogen-absorbing amount and an increase, a decrease, or are-increase of reversible hydrogen-absorbing capacity. The pattern ofchanges in the hydrogenation properties corresponded to a relative ratiobetween BCC phase of Ti--V having no reversible hydrogenabsorption/desorption activity and Ti--V--Mn phase of C-14 hexagonalstructure having reversible hydrogen absorption/desorption activity, ina view of the data obtained from the X-ray diffraction studies.

On the other hand, as shown in Table 1, if an atomic ratio of Mn islower than 0.03, BCC phase dominates independently on the substitutionof V, which results in a remarkable decrease of the reversiblehydrogen-absorbing capacity. Accordingly, in order to improve thehydrogenation properties(a very small amount of reversible hydrogen andhigh pressure) of the Ti--Mn alloy system, e.g., Ti₀.6 Mn₀.4 alloy,having a very large hydrogen-absorbing amount and a lowhydrogen-absorbing capacity, Mn and V elements have to be substitutedwith other elements to elevate the ratio of C14-hexagonal phase.

Reference Example 2: Ti--V--Mn--Ni alloy system

Hydrogenation properties of the Ti--V--Mn alloy system in ReferenceExample 1, in the course of solid-gas reaction, were investigated. As aresult, it was found that: hydrogen-absorption/desorption in theelectrolyte did not occur, while hydrogenation properties of the alloyswere proper for electrode materials. The said phenomena occurs since theelements constituting the alloy system, i.e., Ti, V and Mn, play alittle catalytic role in the charge transfer in the electrolyte,atmospheric oxygen is easily oxidized by OH⁻ ion, etc. in theelectrolyte or forms a membrane of the passive state, thus hydrogen isnot absorbed into the alloys but released as gas.

Accordingly, Mn in the Ti--V--Mn alloy system was substituted with Nielement having a relatively high stability and catalytic function in theKOH electrolyte, as the 4th element composing alloy system, to prepareTi--V--Mn--Ni alloy system, and hydrogenation properties and dischargecapacities were investigated. As a result, it was determined that theTi--V--Mn--Ni alloy system can absorb/desorb hydrogen in the electrolyte(see: Table 2).

                  TABLE 2    ______________________________________    Hydrogenation properties and discharge capacities of Ti--V--Mn    and Ti--V--Mn--Ni alloy systems             Properties of solid-             gas reaction             Hydrogen-                    Plateau  Electrochemical             absorbing                    pressure at                             discharge capacity               capacity 30° C.  10    50    Alloy system               (wt. %)  (atm)    1 cycle                                       cycles                                             cycles    ______________________________________    Ti.sub.0.4 V.sub.0.2 Mn.sub.0.4               1.41     0.1-1     0     0     0    Ti.sub.0.4 V.sub.0.2 Mn.sub.0.2 Ni.sub.0.2               0.95     0.5-2.5  183   201   204    Ti.sub.0.2 V.sub.0.4 Mn.sub.0.4               1.62     0.5-1     0     0     0    Ti.sub.0.2 V.sub.0.4 Mn.sub.0.2 Ni.sub.0.2               1.06     0.7-1.6  207   225   227    ______________________________________

Reference Example 3: Ti--Zr--V--Mn--Ni alloy system As can be seen inTable 2, it was found that Ti--V--Mn--Ni alloy system can absorb/desorbhydrogen in the electrolyte, in contrast to the case of Ti--V--Mn alloysystem. However, the alloys were found to be improper for electrodematerials, since their hydrogen-absorbing capacities decreaseddrastically and their plateau pressure increased. The results aredemonstrated in FIGS. 1(A), 1(B) and 1(C). FIGS. 1(A), 1(B) and 1(C)show X-ray diffraction patterns for Ti₀.4 Zr₀.05 V₀.2 Mn₀.2 Ni₀.2, Ti₀.4V₀.2 Mn₀.2 Ni₀.2 and Ti₀.4 V₀.2 Mn₀.4 alloy systems, respectively. Asshown in FIGS. 1(A), 1(B) and 1(C), substitution of Mn with Ni led to anincrease of BCC phase to have a low hydrogen-absorbing capacity, and adecrease of lattice size of C14-hexagonal phase to result in aremarkable reduction of invasive area within the alloys permittingabsorption of hydrogen. Thus, the equilibrium pressure increased.Therefore, in order to improve the properties of Ti--V--Mn--Ni alloysystem, Ti was substituted partially with Zr element as the 5th element,or a small amount of Zr element was added. Then, changes in thehydrogen-absorbing capacity and hydrogen equilibrium pressure wereinvestigated and summarized in Table 3 below.

                  TABLE 3    ______________________________________    Hydrogenation properties, discharge capacities and microstructures of    Ti--V--Mn--Ni and Ti--Zr--V--Mn--Ni alloy systems                  Hydrogen-                 BCC/                  absorbing                           Equilibrium                                     Discharge                                            C14                  capacity pressure  capacity                                            phase    Alloy system  (wt. %)  (atm)     (mAh/g)                                            ratio    ______________________________________    Ti.sub.0.4 V.sub.0.2 Mn.sub.0.2 Ni.sub.0.2                  0.95     0.5-1.5   204    0.85    Ti.sub.0.4 Zr.sub.0.05 V.sub.0.2 Mn.sub.0.2 Ni.sub.0.2                  1.42     0.3-0.6   372    0.09    Ti.sub.0.3 Zr.sub.0.05 V.sub.0.2 Mn.sub.0.2 Ni.sub.0.2                  1.32     0.4-0.6   335    0.22    Ti.sub.0.2 V.sub.0.4 Mn.sub.0.2 Ni.sub.0.2                  1.06     0.7-1.6   227    0.73    Ti.sub.0.2 Zr.sub.0.05 V.sub.0.4 Mn.sub.0.2 Ni.sub.0.15                  1.52     0.03-0.4  393    0.05    Ti.sub.0.2 Zr.sub.0.05 V.sub.0.4 Mn.sub.0.15 Ni.sub.0.2                  1.5      0.1-0.3   390    0.06    ______________________________________

As can be seen in Table 3, it was found that the hydrogen-absorbingcapacity increased and hydrogen equilibrium pressure was controlled at arange of 0.01 to 1 atm at room temperature, when Ti element in theTi--V--Mn--Ni alloy system was substituted partially with Zr element asthe 5th element, or a small amount of Zr element was added to thealloys. As shown in FIGS. 1(A), 1(B) and 1(C) showing X-ray diffractionpatterns, the said improvement resulted from a re-decrease of BCC phaseand an increase of lattice size.

Example 1

Hydrogen-absorbing Ti--Zr--V--Mn--Ni-M alloy system

Based on the results obtained in Reference Example 3 that an alloyhaving low BCC phase and high C14-hexagonal Laves phase, likeTi--Zr--V--Mn--Ni alloy system, has good hydrogenation properties and ahigh discharge capacity, Ti--Zr--V--Mn--Ni alloy system which wasmodified to have optimum composition and Ti--Zr--V--Mn--Ni--M alloysystem which was prepared by adding the 6th and 7th elements to theTi--Zr--V--Mn--Ni alloy system were prepared, respectively. Then,changes in the crystalline structure, hydrogenation properties anddischarge capacities were investigated, whose results were summarized inTable 4 below.

                                      TABLE 4    __________________________________________________________________________    Hydrogen-absorbing capacities, discharge capacities and lattice constants    of hydrogen-absorbing    Ti--Zr--V--Mn--Ni and Ti--Zr--V--Mn--Ni--M alloy systems                            Solid-gas                                    Electro-                            reaction                                    chemical    Hydrogen-absorbing Zr--V--Mn--Ni--M alloy system                            Hydrogen-                                    reaction    (M: Cr, Co, Fe, Cu, Al, Si, Hf, Nb, Mo and R.E.)                            absorbing                                    Discharge                                         Lattice    Ti  Zr  V   Mn  Ni  M   capacity                                    capacity                                         constant    (at. %)        (at. %)            (at. %)                (at. %)                    (at. %)                        (at. %)                            (wt. % H/alloy g)                                    (mAh/g)                                         a(Å)                                             c(Å)    __________________________________________________________________________    20  13  20  17  30  0   1.9     485  4.946                                             8.060    20  14  20  11  35  0   1.77    464  4.925                                             8.019    20  4   40  15  15  5(Co)                            1.7     451  4.921                                             8.034    23  10  20  17  30  0   1.8     477  4.910                                             8.032    26  7   24  20  23  0   1.58    407  4.902                                             8.012    26  7   24  10  20  10(Cr)                            1.8     475  4.94                                             8.052    26  7   24  10  25  5(Hf,                            1.6-1.65                                    410-416                                         4.925 ±                                             8.033 ±                        Mo,Nb)           0.01                                             0.01    27  7   17  17  32  0   1.7     449  4.920                                             8.013    27  7   18  17  31  0   1.73    454  4.918                                             8.008    27  7   21  14  27  4(Cr)                            1.8     465  4.928                                             8.028    27  7   21  14  27  4(Co)                            1.63    423  4.931                                             8.041    27  7   21  14  27  4(Cu)                            1.56    403  4.916                                             7.988    27  7   21  14  27  4(Al,                            1.5-1.51                                    400-405                                         4.942 ±                                             8.044 ±                        Si)              0.01                                             0.01    27  7   21  14  27  4(Fe)                            1.55    406  4.945                                             8.029    27  7   20  13  27  6(Co)                            1.75    456  4.924                                             8.020    28  7   22  18  25  0   1.82    481  5.004                                             8.168    28  7   20  15  25  5   1.74-1.77                                    450-460                                         4.945 ±                                             8.037 ±                        (R.E.)           0.01                                             0.01    28  7   22  10  25  8(Cr)                            1.88    495  4.93                                             8.036    28  7   22  10  25  8(Al)                            1.65    430  4.925                                             8.022    30  3   17  17  33  0   1.51    400  4.913                                             8.003    30  3   20  17  30  0   1.7     450  4.898                                             7.972    35  4   18  30  13  0   1.63    426  4.932                                             8.060    35  5   15  20  20  15(Cr)                            1.54    400  4.912                                             8.027    __________________________________________________________________________

As can be seen in Table 4, it was found that Ti--Zr--V--Mn--Ni alloysystem or Ti--Zr--V--Mn--Ni--M alloy system(M=Cr, Co, Fe, Cu, Al, Si,Hf, Nb, Mo and R.E. (the rare-earth elements such as La, Ce, Pr, Nd andSm)) of the invention had C14-hexagonal crystalline structures of singlephase and discharge capacities of 400 mAh/g or more. FIG. 2 shows X-raydiffraction pattern for Ti₀.26 Zr₀.07 V₀.15 Mn₀.2 Ni₀.34 alloy system,which is a preferred embodiment of the invention, which confirms thatthe alloy had C14-hexagonal crystalline structure of single phase.

As clearly illustrated and demonstrated above, Ti--Zr--V--Mn--Ni--Malloy system of the invention, comprises basic elements of Ti, Zr, V, Mnand Ni and at least one optional element such as Cr, Co, Fe, Cu, Al, Si,Hf, Nb, Mo and R.E.(the rare-earth elements such as La, Ce, Pr, Nd andSm)) as the 6th and 7th elements in an amount of 0 to 10 at. %. Thealloy system has low molar molecular weight of 50 to 65 g/mol,C14-hexagonal crystalline structure of single phase, lattice constant ofa: 4.902-5.004 Å and c: 7.972-8.168 Å, ultra high discharge capacity of400 mAh/g or more, which can be employed as an anode material of aNi--MH secondary battery.

What is claimed is:
 1. A hydrogen-absorbing Ti-based alloy havingC14-hexagonal crystalline structure of single phase, which isrepresented as a following general formula (I):

    Ti.sub.A Zr.sub.B V.sub.C Mn.sub.D Ni.sub.E                (I)

wherein,A, B, C, D and E have atomic ratios ranging 0.2≦A≦0.35,0.03≦B≦0.15, 0.2≦C≦0.4, 0.08≦D≦0.2 and 0.13≦E≦0.35, with the provisothat A+B+C+D+E=1 and A+B≦0.45.
 2. The hydrogen-absorbing Ti-based alloyof claim 1 which is selected from the group consisting of:Ti₀.26 Zr₀.07V₀.24 Mn₀.20 Ni₀.23 or Ti₀.28 Zr₀.07 V₀.22 Mn₀.18 Ni₀.25.
 3. Ahydrogen-absorbing Ti-based alloy having C14-hexagonal crystallinestructure of single phase, which is represented as a following generalformula (II):

    Ti.sub.A Zr.sub.B V.sub.C Mn.sub.D Ni.sub.E M.sub.F        (II)

wherein,M represents at least one metal which is selected from the groupconsisting of Cr, Co, Fe, Cu, Al, Si, Hf, Nb, Mo and R.E. (rare-earthelements); and, A, B, C, D, E and F have atomic ratios ranging0.2≦A≦0.35, 0.03≦B≦0.15, 0.2≦C≦0.4 0.08≦D≦0.2 and 0.13≦E≦0.35 and0≦F≦0.1, respectively, with the proviso that A+B+C+D+E+F=1 and A+B≦0.45.4. The hydrogen-absorbing Ti-based alloy of claim 3 which is selectedfrom the group consisting ofTi₀.20 Zr₀.04 V₀.40 Mn₀.15 Ni₀.15 Co₀.05 ;Ti₀.26 Zr₀.07 V₀.24 Mn₀.10 Ni₀.20 Cr₀.10 ; Ti₀.26 Zr₀.04 V₀.24 Mn₀.10Ni₀.25 Hf₀.05 ; Ti₀.26 Zr₀.07 V₀.24 Mn₀.10 Ni₀.25 Mo₀.05 ; Ti₀.26 Zr₀.07V₀.24 Mn₀.10 Ni₀.25 Nb₀.05 ; Ti₀.27 Zr₀.07 V₀.21 Mn₀.14 Ni₀.27 Cr₀.04 ;Ti₀.27 Zr₀.07 V₀.21 Mn₀.14 Ni₀.27 Co₀.04 ; Ti₀.27 Zr₀.07 V₀.21 Mn₀.14Ni₀.27 Cu₀.04 ; Ti₀.27 Zr₀.07 V₀.21 Mn₀.14 Ni₀.27 Al₀.04 ; and, Ti₀.27Zr₀.07 V₀.21 Mn₀.14 Ni₀.27 Si₀.04. Ti₀.27 Zr₀.07 V₀.21 Mn₀.14 Ni₀.27Fe₀.04 ; Ti₀.27 Zr₀.07 V₀.21 Mn₀.13 Ni₀.27 Co₀.06 ; Ti₀.28 Zr₀.07 V₀.20Mn₀.15 Ni₀.25 R.E.₀.05 ; Ti₀.28 Zr₀.07 V₀.22 Mn₀.10 Ni₀.25 Cr₀.04 ; and,Ti₀.28 Zr₀.07 V₀.22 Mn₀.10 Ni₀.25 Al₀.08.